Attenuators are passive circuits commonly used to introduce a power loss between a source circuit and a load circuit. Their purpose is to match the power level received by the load circuit. They are also sometimes used as an impedance matching circuit.
The most well-known attenuator structures are T, .pi. and bridged-T structures. T and .pi. structures include three impedances, each mounted on one branch of the structure. The value of these impedances will determine the attenuation generated by the structure as well as its characteristic impedance. For its part, the bridged-T structure comprises four impedances. It is a variant of the T structure. It actually comprises an additional impedance which shunts the two impedances of the upper branches of the T.
In order for the attenuation produced by this type of circuit to be electronically variable, it is conventional to use PIN diodes as a variable impedance. The variation in the impedance of a diode is obtained by modifying the direct current which passes through it. In order to obtain variable attenuation, the T, .pi. or bridged-T structure is therefore supplemented by a biasing device intended to set the direct current flowing in the diodes of the structure. An example of a variable attenuation circuit is described in FIG. 1.
This embodiment example is derived from a .pi. structure. It therefore includes three diodes, one diode D12 on the series branch of the .pi. and two diodes D10 and D11 on the two parallel branches of the .pi.. Resistors R10, R11 and R12 are provided in order to make a direct current flow through the diodes D10 and D11. The anode of the diode D10 is therefore connected to a supply terminal Vcc of the circuit via the resistor R10, its cathode is connected to the anode of the diode D11 via the resistor R11 and the cathode of the diode D11 is earthed through the resistor R12. The value of the direct current flowing in the diodes D10 and D11 is therefore fixed by these three resistors. In order to channel the direct current in the current path R10, D10, R11, D11, R12, the circuit includes decoupling capacitors. A first decoupling capacitor C10 is inserted between the input of the circuit and the anode of the diode D10; a second capacitor C11 is inserted between the cathode of the diode D10 and earth; a third capacitor C12 is inserted between the anode of the diode D11 and earth; a fourth capacitor C13 is inserted between the cathode of the diode D11 and the output of the circuit; lastly, a fifth capacitor C14 is inserted between the input of the circuit and the anode of the diode D12. In order to control the attenuation of the circuit electronically, a control voltage Vc is applied on a control input INc of the circuit. This control input is connected to the anode of the diode D12 via a resistor R13 in series with an induction coil L10. The induction coil L10 makes it possible to decouple the control input INc with the rest of the circuit in alternating mode. In order to improve this decoupling, a capacitor C15 is inserted between the coil and earth.
This attenuator circuit operates in the following way: the direct current flowing in the diodes D10 and D11 is fixed by the resistors R10, R11, R12, and is chosen so as to set the impedance of the diodes D10 and D11 to the desired value, and consequently to set the characteristic impedance and the maximum attenuation of the circuit. The control voltage Vc governs the conduction state of the diode D12. So long as the voltage on the anode of the diode D12 is less than (R12*Vcc)/(R10+R11+R12), the diode D12 is off and the attenuation A is a maximum. Above this value, the diode D12 is on and the attenutation decreases. An example of an attenuation curve of the circuit as a function of the control voltage is illustrated in FIG. 2.
An attenuated circuit of this type is defined by various parameters:
its characteristic impedance; PA1 the size of the dynamic range of its control voltage; and PA1 the position of this range. PA1 characterized in that the biasing device includes means for independently setting the characteristic impedance of the circuit, the size of the dynamic range of the control voltage and the position of the said dynamic range. PA1 and the biasing device consists of a differential pair, a first branch of which is connected to the cathode of the first diode and a second branch of which is connected to the cathode of the second diode, the anode of the said first diode being connected to a continuous supply terminal, the said differential pair being supplied by a variable current source in order to set the characteristic impedance of the attenuator circuit and receiving, on a first input, a reference voltage intended to position the dynamic range of the control voltage of the circuit and, on a second input, the control voltage by means of a first bridge of variable resistors, which first resistor bridge is intended to set the size of the dynamic range of the control voltage.
In the example in FIG. 2, for a maximum circuit attenuation of 12 decibels, the slope of the curve is -3 decibels per volt, the size of the dynamic range of the control voltage Vc is 4 volts, and this range is centred on the value Vc=5 volts. The size and the central point of this dynamic range make it possible to determine its position.
The main drawback of this type of circuit resides in the setting of these various parameters. Indeed, the system of equations belonging to this circuit shows that these parameters are correlated with one another. The result of this is that it is not possible to choose the value of certain parameters independently of the others.
Another drawback resides in the fact that the resistors R10 and R12 used to fix the direct current in the diodes D10 and D11 modify the AC behaviour of the basic circuit, namely the p structure. In order to limit their effects in AC mode, they are generally chosen in a certain interval of values. This will impose an additional condition for setting the circuit parameters.